This invention was made with Government support under Grant No. DABT 63-92-C-0019 awarded by DARPA, and Grants Nos. ECS-8805866 and ECS-8815775 awarded by the National Science Foundation. The Government has certain rights in the invention.
The present invention is directed to a method for fabricating isolated, joined microelectromechanical (MEM) structures and to tungsten structures fabricated by that method. More particularly, the invention is directed to a novel isolation technology utilizing a selective chemical vapor deposition of tungsten whereby released tungsten structures are fabricated which are both mechanically joined and electrically isolated by a dielectric.
Recent developments in microelectromechanical processes have successfully led to the fabrication of microactuators utilizing processes which have involved bulk or surface micromachining. The most popular surface micromachining process has used polysilicon as the structural layer in which the mechanical structures are formed. In a typical polysilicon process, a sacrificial layer is deposited on a silicon substrate prior to the deposition of the polysilicon layer and mechanical structures are then defined in the polysilicon. The sacrificial layer is then etched partially or completely down to the silicon substrate to free the polysilicon structures.
In bulk micromachining, a silicon substrate is etched and sculpted to leave a structure. This typically has been done using wet chemical etchants to undercut single crystal silicon structures from a silicon wafer. However, such processes are dependent on the crystal orientation within the silicon substrate, and the process is difficult to control. A dry bulk micromachining process which utilizes thermolateral oxidation to completely isolate islands of single crystal silicon is also available for the fabrication of submicron, single crystal silicon movable mechanical structures.
Microactuators produced by the foregoing processes have a biased, movable, released structure, for example in the form of one or more beams or plates which are electrostatically driven by nearby electrodes. Such electrodes may consist of parallel plates adjacent the released structure or may consist of comb-shaped capacitive drive actuators as illustrated, for example, in U.S. Pat. No. 5,179,499 of Noel C. MacDonald et al.
The use of electrodes such as parallel plates or surfaces adjacent movable structures for controlling or sensing the motion thereof is illustrated, for example, in U.S. Pat. No. 5,072,288, issued Dec. 10, 1991 and in U.S. patent application Ser. No. 07/868,138, entitled "Methods of Fabricating Integrated Aligned Tunneling Tip Pairs" of Noel C MacDonald, filed Apr. 14, 1992 and assigned to the assignee of the present application. Comb drives, utilizing interdigitated capacitive plates, are also illustrated in the '138 application. The foregoing patents and application are exemplary of methods for producing microelectromechanical structures and of the structures so produced.
In U.S. Pat. No. 5,179,499, adjacent movable plates are driven by a selected potential applied thereacross to produce relative axial motion between the plates. This microstructure has been found to be extremely useful in controlling the motion of microstructures, including impedances, switches, sensing tips and the like. However, the process utilized to fabricate such a structure involves the formation of released structures in a silicon substrate and then covering such released structures with layers of dielectric material and metal. It is desirable to fabricate such devices in wafers for connection to integrated circuits and to allow incorporation of such devices in different levels of such circuits, and for this purpose it has been found desirable to make the movable structure from tungsten.
U.S. Pat. No. 4,746,621 of David C. Thomas et al, assigned to the assignee of the present application, describes a selective tungsten on silicon process for use in producing patterned integrated circuit metal layers. Patterned CVD silicon dioxide trenches are ion implanted with silicon to permit the fabrication of tungsten microstructures. However, to enable the use of tungsten beams in actuators of the type disclosed in the '499 patent described above, and to allow selective movement of such actuators, it is necessary to provide electrical isolation of the mechanical components and to provide selective biasing of the structure, and for this purpose it is necessary to provide a process by which the microelectromechanical structures can be both mechanically interconnected and electrically isolated, whereby they can be integrated into the surrounding circuitry.